The invention relates to the technical field of motor vehicle rear openings, often called tailgates.
A tailgate (1) generally consists of an outer skin (2), forming a single or double block panel, and incorporating all or some of the outer equipment visible from outside the vehicle (light unit, tailgate or rear window opening control, emblem, wash-wipe system, lock, stops, registration plate, etc.) and an inner box (3). FIG. 1 illustrates a tailgate of this type.
An inner box generally consists of:                an upper portion (4) having an opening designed to accommodate a rear window;        a lower portion (5), of type “solid panel” (unlike the upper portion) whose central area (6) comprises the following technical elements:                    holes or openings for components such as wires, linkages, pipes, attachment interfaces to pass through or be secured to;            attachment interfaces such as bonding tracks or riveting holes, for systems such as motorised locks or window washer motors, etc.;            mechanical strengthening shapes such as ribs or bosses.                        
Most inner boxes in the state of the art are made of steel. A minority of tailgate inner boxes are made of thermoplastic material.
Thermoplastic is known to have a low elastic limit (start of plastic deformation at low stress) and therefore a relatively low breaking strength and may suffer from mechanical fatigue during use. To guarantee the geometrical strength of a thermoplastic inner box throughout its lifetime, despite the numerous opening and closing operations it undergoes, it is known to add one or more large metal reinforcements to the thermoplastic inner box.
Finally, inner boxes made of a thermoset plastic material are also known. This material provides better resistance to deformation, but has the disadvantage of being more dense than the thermoplastic material.
Furthermore, to produce the central technical panel of an inner box made of a thermoset plastic material, it is necessary to rework the box. Under industrial conditions, especially mass production, it is difficult for thermoset plastic material moulding methods to produce parts with recesses (non-through hole, through-hole, opening, etc.), especially if they are numerous and of small dimensions These recesses, necessary for the integration of functions during assembly, must then be made after moulding, by remachining to remove material, which has many disadvantages: more operations in the manufacturing sequence involving intermediate storage, defects and rejects generated by these machining operations, and in particular dust requiring evacuation and efficient filtering to protect the operators, as well as cleaning of parts, and industrial equipment to prevent clogging.
Moreover, machining an inner box necessarily involves elimination of the functionally unnecessary material, and therefore a loss of expensive material. This loss can be even greater when the machining or its related operations (storage, cleaning, etc.) generate defects requiring the entire box to be scrapped.
Some boxes made of thermoplastic material or steel do not require any rework. However, it may be necessary to modify their technical panel (depending on the vehicle type or range type, for example). In this case, a complete inner box must be manufactured.